1. Field of the Invention
The present invention relates to apparatus and method for estimating SOH (State Of Health) of a battery, which is a parameter representing capacity degradation of a battery, and more particularly to apparatus and method for estimating SOH of a battery based on SOC (State Of Charge) that is a parameter representing a residual capacity of a battery.
2. Description of the Related Art
Generally, electric vehicles or hybrid electric vehicles (hereinafter, referred to as electric-driven vehicles) are driven in an electric-driven mode using an electric energy stored in a battery.
A vehicle using a fossil fuel operates an engine using a liquid fuel, so it is not difficult to measure a residual amount of fuel. However, in case of an electric-driven vehicle, it is not easy to accurately measure a residual energy of a battery.
An electric-driven vehicle is moved using an energy charged in a battery, so it is important to check a residual capacity of a battery. Accordingly, techniques for informing a driver of information such as a possible traveling distance by checking SOC of a battery are actively developed.
As an example, there is a method for measuring a voltage of a battery while the battery is charged/discharged, estimating an open-circuit voltage of the battery in an unloading state from the measured voltage, and then mapping SOC corresponding to the estimated open-circuit voltage by referring to a SOC table defining a SOC for each open-circuit voltage. However, when a battery is charged/discharged, the estimated voltage of a battery is significantly different from an actual voltage due to an IR drop effect, so accurate SOC cannot be obtained unless such an error is corrected.
For reference, the IR drop effect means a phenomenon that a voltage is rapidly changed when a battery starts being discharged in connected to a load or starts being charged from an external power source. Namely, a battery voltage rapidly decreases when discharge is initiated, and a voltage rapidly increases when charging is initiated.
As another example, there is a method for estimating SOC of a battery by integrating charging/discharging currents of the battery. When this method is used, SOC accuracy is deteriorated as time goes since measurement errors occurring during the current measuring process are continuously accumulated.
Meanwhile, SOH is another parameter representing a state of a battery, besides the above SOC. SOH is a parameter quantitatively representing a capacity change of a battery caused by an aging effect, and it allows checking how much the capacity of a battery is degraded. Thus, if SOH is checked, a battery may be exchanged at a suitable point of time, and also a charging/discharging capacity of a battery may be controlled according to a use term of the battery to prevent overcharging or overdischarging of the battery.
The change of capacity characteristics of a battery is reflected on the change of internal resistance of the battery, so it is known that SOH can be estimated from internal resistance and temperature of a battery. In other words, capacity of a battery is measured for each internal resistance and temperature of a battery through charging/discharging experiments. Then, the measured capacities are evaluated into relative numerical values based on an initial capacity of the battery to obtain a look-up table for SOH mapping. After that, internal resistance and temperature of a battery under an actual battery use circumstance are measured, and then SOH corresponding to the internal resistance and temperature is mapped from the look-up table to estimate SOH of a battery.
In the above SOH estimating method, the most important thing is how accurately an internal resistance of a battery can be obtained. However, it is in actually impossible to directly measure an internal resistance of a battery while the battery is charged/discharged. Thus, commonly, battery voltage and charging/discharging current are measured to indirectly calculate a battery internal resistance according to Ohm's law. However, since the battery voltage is different from an actual voltage due to the IR drop effect and also the battery current has a measurement error, the internal resistance simply calculated according to the Ohm's law and SOH estimated from the internal resistance does not ensure sufficient reliability.